Larval settlement

BiologySettlement is a crucial step in the life history of a coral. It is about time to switch from a mobile planktonic phase to a sessile one. Therefore, the choice of a suitable settlement place is important and, moreover, a conscious decision. The larva has many sensory cells and exhibits a complex searching behavior to find the most appropriate place to settle (video: larvae on the move).

The Larva's choice is determined by abiotic factors such as light, currents and presence of hard substrate, and biotic factors such as the composition of the biofilm on hand (Let’s talk about the biofilm), the availability of certain CCA (crustose coralline algae) and the presence of competitors. As space is a fiercely contested resource and corals have only one chance to choose the right place―it is of great interest who is living right beside. Competitors such as algae or other corals can be aggressive combatants.

ApplicationOver the last years, we used ceramic substrates, tripods, and special SECORE substrates for research purposes. Both worked just fine to induce larval settlement after the respective treatment. Nevertheless, they do have the disadvantage that they require handling of each substrate by hand―as in any today's restoration work.

Working with sexual coral reproduction, we are able to produce large numbers of larvae. To meet the demands of larger scale restoration and to make its application feasible, we need a corresponding amount of substrates that can be handled bunch-wise in large numbers.

We developed new settlement substrates (SECORE Seeding Units) that self-attach to the reef. These new substrates can be produced and handled en mass (coral restoration). They are made of plain concrete using molds. The tiles are four-armed shaped, forming a tetrahedron. Grooves down their arms promote coral settlement, where the recruits are protected during handling. After their return into the wild, the tiny recruits are there equally protected against predators.

Before offering any settlement substrates to a coral larva, it needs to be conditioned in natural seawater tanks or, best, on the reef. After a few weeks, it has grown the right biofilm and enough CCA to become an attractive settlement substrate. It is favorable to settle several larvae on each substrate to ensure the survival of at least one coral recruit. Fusion of primary polyps while they grow into little colonies strengthens their growth.

Currently, we are applying our new substrates in up-scaling pilot restoration projects in Mexico and in Curacao ('sexual coral reproduction').